It is known to cool or heat the inlet combustion air to gas turbines to obtain a desired enthalpy. An excellent explanation of the relationships of relative humidity, wet and dry bulb temperature, and specific volume of air as they affect enthalpy is provided in Smith, McCloskey and Cates U.S. Pat. No. 5,390,505, which is incorporated herein by reference in its entirety. See particularly the psychrometric chart of FIG. 9.
In addition, the combustion air consumed by a gas turbine may require heating to prevent ice formation as a function of dew point at ambient temperatures below about 43 degrees F. It is also known that, where ambient temperatures are above about 43 degrees F., cooling the combustion air going to a gas turbine will result in increased power output of the gas turbine. Temperature affects air density, and turbine efficiency is in turn affected by the density of the intake air. It is desirable to control the temperature of the heat exchange fluid in the coils of a turbine which contact the incoming air, to achieve greater power output and efficiency and at the same time prevent icing on the outside of a heat exchange coil. Icing on the outside of the coils is quite undesirable, mostly because ice formation can damage the gas turbine if ingested but also because it decreases the heat exchange in the areas affected, and also impedes the flow of air through the intake. The temperature control can be programmed to take into account the factors which affect the desired outcome. As is known in the art, in many cases it may not be necessary to heat the air to a temperature above 32° F.; indeed in many cases (because of the moisture content of the air at the ambient temperature and density) one need only heat the incoming air from −20°, for example, to −10° F., in order to inhibit icing on the outside of the coil.
As illustrated in the above referenced U.S. Pat. No. 5,390,505, the efficiency of the gas turbine may be enhanced by either increasing or decreasing the temperature of the intake air under various circumstances. The air density may vary as a function of the air temperature: “Provision of reduced temperature or increased density air rather than ambient air to a gas turbine-generator generally provides an increase in turbine efficiency and output capacity or generator KW”. Column 7, lines 58-61. The improvement in turbine-generator efficiency is illustrated In FIG. 10 of that patent, where water vapor concentration is considered also as a factor in plotting enthalpy.
Fluid heat transfer coils have been successfully used to cool the intake air, using heat exchange fluids such as water, ethylene glycol solution, propylene glycol solution or alcohol brines in direct or indirect contact with the combustion air. But many of the fluids used in the past, such as ethylene glycol or propylene glycol, are hazardous pollutants and have regulatory classifications. Moreover, many cooling fluids conventionally used in gas turbines, such as the glycols, tend to become very viscous as the working temperature is reduced, which is counterproductive to the purpose of improving heat transfer efficiency for lowering the temperature of the air to increase its density. A highly viscous heat exchange fluid will tend to have a low Reynolds Number—that is, its flow will tend to be laminar rather than turbulent, thus decreasing its heat transfer efficiency. And, more energy will be required to pump it. While plain water has good heat transfer efficiency and viscosity characteristics, its freeze point clearly limits its low temperature acceptability.
DeVault, in U.S. Pat. No. 5,555,738, teaches use of an ammonia water refrigeration system to cool the inlet air of a gas turbine for improved efficiency. Lewis et al, in U.S. Pat. No. 6,195,997, disclose an energy recovery system using a refrigeration loop to cool the inlet air for a gas turbine.
Hallman et al SPE paper # 65616 teaches use of aqueous formates to improve thermal performance of line heaters in gas production and transmission systems. See also Smith et al U.S. patent application Ser. No. 09/788,115 filed Feb. 16, 2001.
It would be desirable to control the temperature of the incoming combustion air in a gas turbine to obtain an optimum power output and efficiency, using a fluid having good heat exchange properties and also a low viscosity at low temperatures.